This invention relates to an exhaust silencer unit for a marine propulsion unit and more particularly to an improved expansion chamber type of silencing device for the exhaust gases of a small planing marine propulsion unit.
It is well known that the exhaust gases of internal combustion engines can, if emitted to the atmosphere unmuffled, create substantial noise. It has, therefore, been the practice to provide some form of silencing device in the exhaust system for such engines. The problems of silencing the exhaust gases of an internal combustion engine are magnified when the engine runs over a wide range of speeds and load conditions since the tuning of the exhaust system for these varying conditions can give rise to substantial problems. In addition, when the exhaust system is used in a marine vehicle, still further problems are encountered because of the relatively small space and confined area in which the exhaust system may be provided. This is particularly true for a small planing craft where the space is limited.
Exhaust silencing is dependent upon and proportional to the path length between the engine and the end of the exhaust path. The longer the exhaust path, the greater the degree of silencing. Any increase in path length, however, usually involves an increase in the volume and area of a watercraft to accommodate the increased path length.
In addition, an expansive apparatus for silencing exhaust is usually bulky and can unbalance a watercraft when concentrated in the spaces available to accommodate exhaust silencers. The exhaust silencer for a watercraft also must provide water locking, that is, the stoppage of water from backflowing into the engine. Some exhaust systems are resistive to water backflow but do not waterlock under the most extreme conditions such as capsizing of the watercraft.
An improved exhaust silencer for the exhaust gases of an internal combustion engine, of high efficiency and having relatively small size, is needed for a marine propulsion unit.
An improved exhaust silencer unit for a marine propulsion unit would have a compact, balanced configuration and yet provide good silencing under a wide range of engine running conditions, and sufficient water locking.
The use of expansion chambers in exhaust systems is well known to achieve good silencing. The expansion chamber is effective to produce the good silencing under a wide range of load conditions and the number of expansion chambers employed in the exhaust system can significantly improve the silencing. However, in connection with marine propulsion units it has been difficult to provide plural expansion chambers due to the relatively small size and compact nature required for the exhaust system.
In addition, it is well known in marine propulsion units to mix the coolant from the engine cooling system with the exhaust gases and discharge them together. This provides a simplicity in the plumbing arrangement for the engine and also the addition of the cooling water to the exhaust gases can improve silencing. However, it must be ensured that the coolant from the engine cooling jacket in addition to the previously mentioned water from the body of water in which the watercraft is operating does not re-enter the engine through the exhaust system which could cause damage to the engine. Therefore, an exhaust system for a marine propulsion unit embodying multiple expansion chambers and which ensures that water cannot re-enter the engine through exhaust ports is necessary.